Abstract: Using the 7 100 absolute first arrivals of P waves and 91 513 relative P arrival times of 726 events at the northeastern margin of the Ordos block since 2009, the 3D fine structure of P wave velocity within the depth of 15km in the crust was inverted by the double difference seismic tomography method. The results show that there exist obvious high-speed continuous bodies in the northwest of the study area, and their lateral areas increase gradually with depth, while the velocity of east and south is relatively low. The velocity inhomogeneity exists and differs at different depths. The lateral differences of velocity are related to seismicity and faults. The 5~15km depth profile shows that earthquakes tend to occur in the area with relatively high velocity or high speed transition zones, which to some extent reflects the fragility of regional crustal media and the strong differential movement of faults in vertical and horizontal directions where the crust body is easy to absorb and store strain energy and generate major earthquakes. A "Y"-shape low-velocity channel is present in the lower crust around Liangcheng, corresponding to the NW-trending Heilaoyao-Shahukou fault set, which may reveal the migration path of the Late Tertiary-Quaternary basalt eruption. The Helingeer M6.2 earthquake in 1976 was related to the formation of the locking section of the thermal welding in this area. The three-dimensional fine structure of P wave velocity presented in this paper provides intuitive seismological evidence for physical and chemical properties of crustal media and the deep tectonic environment of earthquake preparation.

Key words: Double difference tomography, Fine structure of P wave velocity, Northeastern margin of Ordos block, Earthquake relocation, Active tectonics

摘要： 筛选出2009年以来26个台站记录的726条天然地震的7 100条P波到时数据和91 513条相对到时数据，采用双差层析成像方法反演了鄂尔多斯块体东北缘地壳浅层15km深度以内的P波速度三维精细结构。结果显示，研究区西北部存在明显的连续的高速体，高速体的横向面积随着深度逐渐增加，东部和南部则表现为相对低速的分布特征。速度横向不均匀性在各个深度都存在，但在不同深度上又有差异，这种速度横向分布的差异性与地震活动性和断裂构造具有一定的相关性。从0～15km的深度剖面看，地震多发生在相对高速区或者高、低速过渡区，这在一定程度上反映了区域地壳介质体比较脆弱，断裂构造在纵横方向上差异活动比较剧烈，易于吸收并储存应变能，产生了较强的地震活动。凉城一带的地壳下方存在1个"Y"字型的相对高速区通道，并与NW向展布的黑老夭-杀虎口活动断裂组对应，呈现了该区新近纪—第四纪玄武岩喷发的运移轨迹，而1976年和林格尔6.2级地震的发生更是与该部位火山喷发引起热力"焊接"后形成闭锁区段有关。文中给出的P波速度三维精细结构，为揭示鄂尔多斯块体东北缘的地壳介质体的物化性质及地震孕育的深部构造环境提供了直观的地震学依据。

关键词: 双差层析成像, P波速度结构, 鄂尔多斯块体东北缘, 重新定位, 活动构造